This is the In/Out table I've been given:

In|Out

3|8

5|12

9|20

15|32

X|?

Should the bottom of the In/Out table be 2(X + 1)OR2X + 2? Or are both equations correct? Or are neither correct? This is a problem I'm not sure about. 2X + 2 seems to work but I'm not sure about the other one. Thanks for your advice.